1. Field of the Invention
The present invention relates to an electronic device protection circuit for inhibiting an overvoltage exceeding the rated voltage of an electronic device such as a lithium-ion secondary battery and a semiconductor device from being applied thereto so as to protect the electronic device from performance degradation, damage and so on.
Further, the present invention relates to an electronic device protection circuit for protecting an electronic device such as a semiconductor integrated circuit included in, for example, a lithium-ion secondary battery or a cellular phone apparatus from damage resulting from overvoltage or overcurrent.
2. Description of the Related Art
When an overvoltage greater than the rated voltage of an electronic device is applied thereto, it may result in performance degradation in or damage to the electronic device. In order to protect the electronic device from the application of such overvoltage, various protection circuits have been proposed.
Conventionally, as an electronic device protection circuit of this kind, a protection circuit in which, for example, the power source terminals of a semiconductor integrated circuit or input/output terminals (cathode and anode) of a secondary battery are connected in parallel to a zener diode, and a fixed resistor device is provided between the semiconductor integrated circuit or the secondary battery, and external connection terminals where a voltage is externally supplied is typically used.
In recent years, as mobile electronic apparatuses and cellular phones have been developed, lithium-ion secondary batteries have been used as thin secondary batteries which are compact and capable of long-duration discharge. In general, the lithium-ion secondary battery is charged with a voltage slightly greater than its rated discharge voltage. When an excessive charge voltage greater than the rated voltage is applied to the lithium-ion secondary battery, for example, because an abnormal condition in a charger is encountered or a user accidentally connects the battery to an out-of-specification charger, a battery/cell in the battery may generate heat, which may lead degradation or damage therein due to a rise in gas pressure. In order to protect the lithium-ion secondary battery from such degradation and damage, a protection circuit for preventing an overvoltage from being applied to the lithium-ion secondary battery is required.
In accordance with such conventional electronic device protection circuit, when an overvoltage exceeding the breakdown voltage of the zener diode is applied to the external connection terminals of the semiconductor integrated circuit or the secondary battery, the zener diode enters in a state that a current flows therethrough, so that an larger amount of the overcurrent resulting from the overvoltage can flow through the zener diode than through the semiconductor integrated circuit or the secondary battery, and due to the breakdown voltage characteristics of the zener diode, a voltage between terminals of the semiconductor integrated circuit or the secondary battery can be set not to exceed the rated voltage. At this time, as the passage of a large current through a fixed resistor results in a voltage drop, the voltage applied to the semiconductor integrated circuit or the secondary battery can be reduced to the amount determined by the subtraction of the voltage drop by the fixed resistor from the voltage applied to the external connection terminals.
As a technology of the protection circuit for the lithium-ion secondary battery, there is a protection circuit proposed in, for example, Japanese patent laid-open publication Hei 2-87935 (Japanese Patent No. 2720988). In this technology, a zener diode is connected in parallel to a battery/cell in the secondary battery, and a thermal fuse is connected in series to the battery/cell, so that when an overvoltage is applied to the secondary battery, a current flows through the zener diode, which thereby generates heat. As the heat and heat generated in the thermal fuse result in a fusion of the thermal fuse, the overvoltage is prevented from being applied to the battery/cell.
Further, in Japanese utility model publication Hei 6-31345, a technology that in a case where a voltage between a positive terminal and a negative terminal of a secondary battery is detected by a voltage detection circuit, when the voltage detection circuit detects that the voltage rises to become an overvoltage greater than the rated voltage, a heat generation switching device turns on to generate heat, which is applied to a thermal sensitive interrupting device to be brought into an interrupting state, so that the overvoltage is prevented from being applied to the secondary battery has been proposed.
Further, other various technologies such that a charge voltage is applied to a secondary battery through a protection circuit with a complicated circuit configuration using a thermistor or a digital circuit device, and that a bimetal is used instead of a fuse have been proposed.
However, the foregoing technologies which have conventionally proposed have some problems such that when a further overvoltage is applied, the protection circuit itself may be damaged so as to become inoperable, or may generate heat which thereby causes degradation or damage in the battery/cell.
Further, there is a problem that in the foregoing conventional electronic device protection circuit. When an overvoltage is continuously applied to the external connection terminals, or when an overvoltage which may result in an increase in current to greater than the maximum allowable current of the zener diode is applied, the zener diode abnormally generates heat, which may damage circuit devices or semiconductor integrated circuits on its periphery, or the zener diode itself due to overheating.
In the technology proposed in Japanese patent laid-open publication Hei 2-87935, for example, when a user accidentally connects the secondary battery to the out-of-specification charger, and an excessive charge voltage which is much greater than the rated voltage is applied to the secondary battery, the thermal fuse generates heat, resulting in the fusion of the thermal fuse, so that the overvoltage can be prevented from being applied to the secondary battery. However, when an overcurrent much higher than the rated current due to the overvoltage continuously flows through the zener diode, the zener diode itself will be overheated, which may result in the degradation in or damage to the battery/cell.
Moreover, before the fusion of the thermal fuse, when the zener diode is damaged and fixedly bypasses a current, so that the battery/cell may be useless as a secondary battery because the battery/cell may not be charged permanently, or the positive and negative terminals of the battery/cell may be always short-circuited. Alternatively, when the zener diode is damaged so as to become an electrical resistor, the current continuously flows through the damaged zener diode. As a result, the zener diode itself is overheated, which thereby may result in degradation in or damage to the battery/cell.
In the technology proposed in Japanese utility model publication No. Hei 6-31345, for example, in a case where the excessive charge voltage which is much greater than the rated voltage is accidentally applied to the secondary battery, at the moment when the excessive charge voltage is applied, a returnable type thermal sensitive device has not been brought into an interrupting state yet, so an overcurrent flows through at the moment, resulting in damage to a heat generation switching device. Thereby, even if the excessive charge voltage is applied, the excessive charge voltage cannot be detected and the thermal sensitive device does not function. Accordingly, as the excessive charge voltage is continuously applied to the secondary battery, the secondary battery generates heat, which thereby may result in degradation in or damage to the secondary battery. Alternatively, when the heat generation switching device is damaged and brought into an constant on state due to the application of excessive charge voltage, the current continuously flows through, therefore, the heat generation switching device is overheated, which thereby may result in degradation in or damage to the battery/cell.
In order to prevent the foregoing protection circuits which have been conventionally proposed from being damaged, it is deemed effective to set the rated current (allowable current capacity) of the heat generation switching device or the zener diode larger. However, when the rated current becomes larger, it is difficult to detect the application of a charge voltage greater than the rated voltage or to reliably carry out an interruption or fusion in response to the voltage.
Alternatively, in the case of a configuration that only the thermal fuse is connected in series to the secondary battery without using any protection circuit, there is a problem that as an overcurrent due to an overvoltage continuously flows through the secondary battery before the fusion of the thermal fuse, the secondary battery may be degraded or generate heat in the meantime. Likewise, in a case where the bimetal is used instead of the fuse, there is a problem that as an overcurrent continuously flows through the secondary battery until the bimetal interrupts the current, the secondary battery may be degraded or generate heat.
Further, in order to prevent overheating or damage resulting from the overcurrent flow through the zener diode, the electrical resistance of a fixed resistor device can be set to be larger so as to inhibit the overcurrent flow in spite of the application of the overvoltage. However, due to a voltage drop by the larger electrical resistance, even if a normal voltage less than the rated voltage is applied to the external connection terminals, a low voltage less than a practical allowable range is applied to the semiconductor integrated circuit. Thus, even if the normal voltage is applied, the semiconductor integrated circuit does not operate properly.
Therefore, there is a problem that the voltage range that the protection circuit effectively functions is limited to a predetermined small range, so that the protection circuit cannot work effectively in a wider range other than the small predetermined range.
For example, in the case of a conventional electronic device protection circuit designed to protect a semiconductor integrated circuit with the rated voltage of 5 V from the application of a voltage up to 20 V, if an overvoltage greater than 32 V is applied to the semiconductor integrated circuit, the zener diode is overheated, which thereby may result in damage to the zener diode itself or circuits on its periphery. On the other hand, in a case where a fixed resistor device with larger resistance is replaced in order not to overheat the zener diode in spite of the application of an overvoltage exceeding 20 V such as 32 V, even though the rated voltage of 5 V is externally applied, due to a voltage drop in the fixed resistor device by the consumed current of the semiconductor integrated circuit, a low voltage substantially less than the rated voltage is applied to the semiconductor integrated circuit, which thereby does not operate properly.
Further, in the case of a protection circuit using a thermistor or a digital circuit device, its circuit configuration tends to be complicated, so it is difficult to downsize the protection circuit or reduce its cost.
Moreover, it is considered to use a constant-voltage circuit as an electronic device protection circuit, but as in the above case, there is a problem that since the voltage range that the constant-voltage circuit can effectively function as a protection circuit is limited to a predetermined small range, and the electrical parts used in the circuit are semiconductor integrated circuits or devices which are comparatively weak against the application of overvoltage, another protection circuit is required for protecting the electronic device protection circuit, so that the circuit configuration becomes complex.
Still further, there is the same problem as described above in a protection circuit used for preventing an overvoltage from being applied not only to the secondary battery on charge, but also to, for example, a semiconductor device or a liquid crystal display device.
The present invention has been achieved in view of above problems, and it is an object of the present invention to provide an electronic device protection circuit which can reliably inhibit an overvoltage from being applied to a secondary battery, a semiconductor device or any other electronic device, and can prevent the protection circuit itself from being degraded or damaged.
An electronic device protection circuit according to the present invention for inhibiting a voltage exceeding the rated voltage of an electronic device from being applied positive and negative voltage input terminals of the electronic device comprises a device having characteristics of making an increase in current when a voltage set to greater than the rated voltage is applied thereto, such as a zener diode, a device having characteristics of inhibiting a current by an increase in its electrical resistance in response to at least either of an increase in current or a rise in temperature, such as a posistor, and a configuration in which the devices connected in series are connected in parallel to a battery on the protection circuit.
An electronic device protection circuit according to a first invention comprises a posistor having characteristics of making an increase in its electrical resistance in response to an increase in current, of which an end is connected to either of the voltage input terminals of the electronic device, a thermal fuse having characteristics that the flow of a current therethrough by applying the voltage exceeding the rated voltage causes a rise in temperature, resulting in the fusion thereof, of which an end is connected to the other voltage input terminal of the electronic device, a zener diode having characteristics that applying a voltage exceeding its breakdown voltage which is set to greater than the rated voltage results in an increase in current, which is connected to the other end of the posistor and the other end of the thermal fuse, and a configuration in which as the thermal fuse and the zener diode are provided to be able to conduct heat each other, applying an overvoltage greater than the rated voltage to the voltage input terminals of the electronic device makes a current resulting in the overvoltage flow through, so that the zener diode generates heat, which thereby accelerates the fusion of the thermal fuse, and the current resulting from the overvoltage is bypassed to the zener diode and the posistor to reduce the current flow through the electronic device, and further, when the electrical resistance of the posistor increases, the increased electrical resistance reduces the current flow through the zener diode and the posistor to less than their rated currents.
The electronic device protection circuit according to a second invention comprises a zener diode having characteristics that applying a voltage greater than its breakdown voltage which is set to greater than the rated voltage results in an increase in current, which is connected in parallel to the electronic device and a posistor having characteristics of inhibiting a current by an increase in its electrical resistance in response to at least either of an increase in current or a rise in temperature, which is connected in series to the zener diode.
The electronic device protection circuit is configured to be capable of protecting the electronic device from being damaged or degraded resulting from the application of an overvoltage, and even if the overvoltage is continuously applied to the external connection terminals, or an overvoltage which may result in an increase in current to greater than the maximum allowable current of the zener diode is applied, to be capable of coping with a wide range of the overvoltage without overheating or damaging the zener diode.
Further, another electronic device protection circuit according to the first invention comprises a zener diode having characteristics that applying a voltage exceeding its breakdown voltage which is set to greater than the rated voltage results in an increase in a current, of which an end is connected to either of the voltage input terminals of an electronic device, a thermal fuse having characteristics that the flow of a current therethrough by applying the voltage exceeding the rated voltage causes a rise in temperature, resulting in the fusion thereof, of which an end is connected to the other voltage input terminal of the electronic device, a posistor having characteristics of making an increase in its electrical resistance in response to an increase in current, in which the Curie point, which is a temperature that the electrical resistance of the posistor nonlinearly and steeply increases, is set to higher than the fusion temperature of the thermal fuse, and which is connected to the other end of the zener diode and the other end of the thermal fuse, and a configuration in which as the thermal fuse, the posistor and the zener diode are provided to be able to conduct heat one another, applying an overvoltage greater than the rated voltage to the voltage input terminals of the electronic device makes a current resulting in the overvoltage flow through, so that the zener diode and the posistor generate heat, which thereby accelerates the fusion of the thermal fuse, and the current resulting from the overvoltage is bypassed through the zener diode and the posistor to reduce the current flow through the electronic device, and further, when the electrical resistance of the posistor increases, the increased electrical resistance reduces the current flow through the zener diode and the posistor to less than their rated currents.
In the electronic device protection circuit according to the first invention, when an overvoltage greater than the rated voltage is applied to the voltage input terminals of the electronic device, a current resulting from the overvoltage flows through the zener diode, so that the zener diode generates heat, which thereby accelerates the fusion of the thermal fuse, so the fusion of the thermal fuse is reliably carried out. Moreover, by bypassing the current resulting from the application of the overvoltage to the zener diode and the posistor, the flow of the current through the electronic device is reduced. Further, when the electrical resistance of the posistor increases by the continuous flow of the overcurrent resulting from the application of the overvoltage, the increased electrical resistance reduces the current flow through the zener diode and the posistor to less than their rated currents.
In still another electronic device protection circuit according to the first invention, when an overvoltage greater than the rated voltage is applied to the voltage input terminals of the electronic device, a current resulting from the overvoltage flows through the zener diode, so that the posistor and the zener diode generate heat, which thereby accelerates the fusion of the thermal fuse, so the fusion of the thermal fuse is reliably carried out. Moreover, by bypassing the current resulting from the application of the overvoltage to the zener diode and the posistor, the flow of the current through the electronic device is reduced. Further, when the electrical resistance of the posistor increases by the continuous flow of the overcurrent through the posistor, the increased electrical resistance reduces the current flow through the zener diode and the posistor to less than their rated currents.
Further, a thermal fuse of which the fusion temperature is higher than that of the thermal fuse may be also placed between the zener diode and the posistor, so that after the fusion of the thermal fuse, the fusion of the thermal fuse of which the fusion temperature is higher may be carried out to interrupt the current flow through the zener diode and the posistor.
Moreover, a thermostat may be provided instead of the thermal fuse. When the application of a voltage exceeding the rated voltage to positive and negative voltage input terminals of the electronic device causes beat generation in the zener diode and the posistor, which thereby results in a rise in the temperatures thereof to higher than predetermined temperatures, the thermostat interrupts the current flow. When the temperatures drop after the interruption of the current flow, for example, due to no application of the overvoltage, the thermostat automatically returns to the initial conduction state, so that the thermostat can be repeatedly used.
Alternatively, a thermostat of which the fusion temperature is higher than that of the thermostat may be also provided instead of the thermal fuse of which the fusion temperature is higher, so that when the temperature drops due to no application of the overvoltage, the thermostat can return to the initial conduction state.
The electronic device protection circuit according to the first invention is suitable for, for example, lithium-ion secondary battery, lithium polymer secondary battery, lithium metal secondary battery, NiCad secondary battery, nickel metal hydride battery and so on. However, it is obvious that it is suitable not only for those described above but also for the use of protecting an electronic device such as a semiconductor device and a display device which, when a voltage greater than its rated voltage is applied thereto, may result in performance degradation or damage.
The electronic device protection circuit according to the second invention for inhibiting a voltage exceeding the rated voltage of an electronic device from being applied thereto, comprises a zener diode having characteristics that applying a voltage greater than its breakdown voltage which is set to greater than the rated voltage results in an increase in current, which is connected in parallel to the electronic device and a posistor having characteristics of inhibiting a current by an increase in its electrical resistance in response to at least either of an increase in current or a rise in temperature, which is connected in series to the zener diode.
In the electronic device protection circuit according to the second invention, when an overvoltage exceeding the rated voltage of the electronic device is applied, a zener diode which has been virtually out of conduction changes into a state that current flows therethrough, and most of a large current resulting from the application of the overvoltage flows through the zener diode so as to reduce the current flow through the electronic device. At this time, due to the basic characteristic of the zener diode, when a voltage greater than the breakdown voltage is applied, in spite of the application of a voltage greater than the voltage, the voltage between the terminals of the zener diode is locked on at the value of the breakdown voltage so as to maintain almost constant, and consequently the voltage between the terminals of the electronic device connected in parallel to the zener diode maintains almost constant at the value of the breakdown voltage so as to inhibit the voltage from becoming a greater overvoltage.
When a higher voltage is applied, a higher current flows through the zener diode, and when the overvoltage is continuously applied, the zener diode continuously generates heat. However, as a posistor is connected in series to the zener diode, the electrical resistance of the posistor increases in response to an increase in current so as to reduce the current to less than the maximum allowable current. Alternatively, when the zener diode generates heat, the heat raises the temperature of the posistor, resulting in an increase in the electrical resistance of the posistor, so the current is reduced to less than the maximum allowable current of the zener diode.
In order to reduce the current to less than the maximum allowable current of the zener diode as described above, it is preferable to set the rated current of the posistor to less than the maximum allowable current of the zener diode. Further, the posistor and the zener diode may be provided to be able to conduct heat each other, so that when an overvoltage exceeding the rated voltage is applied to the electronic device, the heat generated in the zener diode accelerates an increase in the electrical resistance of the posistor to more reliably prevent overheating or damage resulting from the current flow through the zener diode.
Other and further objects, features and advantages of the invention will appear more fully from the following description.